3. A student observes that a 126 mL dry flask with a foil cap weighs 50.162 g. T

Question

3. A student observes that a 126 mL dry flask with a foil cap weighs 50.162 g. The student then adds 5 mL of an unknown volatile liquid and heats the flask in a boiling water bath at 99.2 degree C until all liquid disappears. The flask, foil, and unknown after heating that is dry and cool weigh 50.610 g. The barometric pressure in the laboratory is 743.3 mm Hg; the laboratory temperature averages 22.3 degree C; the vapor pressure of the unknown liquid is 160 mmHg at 20 degree C, and the density of dry air at STP is 1.2929 g/L. a) What are the experimental conditions of T and P that will be used to calculate the mass of air in the flask?

Explanation / Answer

a. Experimental P and T is the laboratory temperature and pressure.

T = 22.3C= 295.3K

P = 743.3mmHg = 0.98atm = 99298pascal

b. Density() = MP/RT ....(M for air = 29Kg/Kg.mole)

of air at 295.3K = 29(Kg/Kg.mole) 99298(N/m2) / 8314.5 (Kg/Kgmole) 295.3K

=1.17 kg/m3 = 1.17 g/L

c. mass of air = 0.126 L * 1.17 g/L

=0.147 g

d. Mass of air after cooling = 0.121 L * 1.17 g/L = 0.141 g =0.141/29 = 0.005 mole

Partial pressure of air = nRT/V = 0.005 * 0.082 * 295.3 /0.126 = 0.96 atm

e. Mass of air later= 0.121 L * 1.17 g/L = 0.141 g

f. Air lost = 0.147 - 0.141 = 0.006 g

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